| Resistance to fire blight, caused by Erwinia amylovora, is found in the pear cultivars Old Home and Seckel. To identify polymerase chain reaction (PCR)-based markers linked to this resistance, resistance gene analogs (RGAs) and random amplified polymorphic DNAs (RAPDs) were amplified from these fire blight resistant cultivars and a susceptible cultivar, Bartlett. Specific primers were designed, based on the sequences of 14 RGAs and 4 polymorphic RAPD bands unique to the pear cultivar Old Home or Seckel, but no polymorphisms could be found between these cultivars and Bartlett. This was perhaps due to the 92--93% similarity between these pear cultivars, based on the RAPD analysis. Other studies have typically shown 75--91% similarities between P. communis cultivars. To obtain transgenic apples resistant to E. amylovora, a gene for the cecropin analog, MB39, under the control of the CaMV35S promoter, was transformed into M. x domestica cv. McIntosh. Pure MB39 peptide showed high antibacterial activity against E. amylovora. Seven transgenic lines were generated containing MB39, and stem inoculation with E. amylovora showed that they had a 42--55% reduction in the relative length of stem necrosis compared to non-transformed plants. A PCR-based marker was also sought for the Vf resistance gene for apple scab, caused by Venturia inaequalis , which originated in Malus foribunda 821 and has been bred into many cultivated apple cultivars. Cloning of the Vf locus has resulted in the description of at least four genes, HcrVf1--HcrVf4. PCR primers were designed from conserved regions of these genes to yield length polymorphic PCR products. A total of 19 cultivars of M. x domestica containing the Vf gene and 13 cultivars without Vf resistance were examined by PCR. A PCR product corresponding to Vfa4 was the only one of these Vf genes amplified exclusively from cultivars with Vf scab resistance. The same PCR product was amplified from M. floribunda 821. This indicated that Vfa4 is the most reliable marker for Vf scab resistance among the genes described thus far at the Vf locus. |
